Imaging systems, such as computer-to-plate (CTP) imaging systems are known in the art and are used to record an image on a lithographic printing plate precursor. Such precursors comprise a planar substrate typically composed of aluminum that has a hydrophilic surface on which one or more radiation-sensitive imageable layers are disposed. In lithographic printing, lithographic ink receptive regions, known as image areas, are generated on the hydrophilic surface of the planar substrate. When the printing plate surface is moistened with water and a lithographic printing ink is applied, hydrophilic regions retain the water and repel the lithographic printing ink, and the lithographic ink receptive image regions accept the lithographic printing ink and repel the water. The lithographic printing ink is transferred to the surface of a material upon which the image is to be reproduced, perhaps with the use of a blanket roller.
Lithographic printing plate precursors are considered either “positive-working” or “negative-working.” Positive-working lithographic printing plates precursors are designed with one or more radiation-sensitive layers such that upon imagewise exposure to suitable radiation, the exposed regions of the layers become more alkaline solution soluble and can be removed during processing to leave the non-exposed regions that accept lithographic ink for printing.
In contrast, negative-working lithographic printing plate precursors are designed with a radiation-sensitive layer such that upon imagewise exposure to suitable radiation, the exposed regions of the layer are hardened and become resistant to removal during processing, while the non-exposed regions are removable during processing that can be carried out on-press during lithographic printing in the presence of a fountain solution, lithographic printing ink, or both.
In the current state of the art in the lithographic printing industry, lithographic printing plate precursors are usually imagewise exposed to imaging radiation such as infrared radiation using lasers in an imaging device commonly known as a platesetter (for CTP imaging) before additional processing (development) to remove unwanted materials from the imaged precursors. Manufacturers typically provide precursors in “stacks” of equivalently-sized elements, perhaps separated from each other by interleaf paper. A stack of precursors can be delivered on a pallet or other structure that provides support and simplifies conveyance. Alternatively, a stack of precursors can be held within a carton, cassette, or other protective enclosure that provides desired protection and orientation for use.
Many imaging systems provide integrated storage facilities for a quantity (stack) of lithographic printing plate precursors to be used and provide automated mechanisms or apparatus for selecting and loading each precursor for imaging. For example, a platesetter can be used with an autoloader (or loading apparatus or plate feeding apparatus) that automatically picks up an individual precursor from a stack and loads it onto an imaging drum where each precursor is appropriately imagewise exposed with suitable radiation. Such a combination of features in an imaging apparatus provides for considerable automation and high throughput for certain high production printing jobs such as the printing of newsprint. The stacks of multiple lithographic printing plate precursors can be arranged in a supply area near the platesetter, ready for loading using the autoloader.
U.S. Pat. No. 6,840,176 (Armoni) describes a CTP system comprising imaging units and a stack of lithographic printing plate precursors aligned for automatic loading into the imaging units (platesetters). An apparatus for loading lithographic printing plates is also described in U.S. Pat. No. 8,739,702 (Korolik et al.) and a plate handling system for this purpose is described in U.S. Pat. No. 7,861,940 (Cummings et al.).
In such automatic printing operations, the lithographic printing plate precursors are often stored for an extended period near the platesetter without any covering to protect the radiation-sensitive imageable layer in each precursor from ambient conditions.
It has been found that certain lithographic printing plate precursors such as negative-working lithographic printing plate precursors, are susceptible to loss of imaging sensitivity when exposed to ambient ozone without a protective covering near or inside a platesetter. Ambient ozone content is typically around 50 ppb and can be higher near electric equipment because of ozone generated by such equipment. Having discovered this problem from the action of ozone, there is a need to solve it for the lithographic printing industry so that imaging sensitivity is not lost and high-speed lithographic printing of newsprint can be achieved efficiently.